1. Field of the Invention
This invention relates generally to an orthomode coupler for a cellular communications system and, more particularly, to a tapered orthomode coupler for a cellular communications system that allows for dual sense polarization for both transmission and reception frequency bands.
2. Discussion of the Related Art
Various communications systems, such as certain cellular telephone systems, cable television systems, internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and then retransmitted by the satellite to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The uplink and downlink signals are typically transmitted at different frequency bandwidths. For example, the uplink communications signal may be transmitted at 30 GHz and the downlink communications signal may be transmitted at 20 GHz.
The satellite is equipped with an antenna system including a configuration of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth. Typically, the antenna system includes one or more arrays of feed horns, where each feed horn array includes an antenna reflector for collecting and directing the signals. In order to reduce weight and conserve satellite real estate, some satellite communications systems use the same antenna system and array of feed horns to receive the uplink signals and transmit the downlink signals. Combining satellite uplink signal reception and downlink signal transmission functions for a particular coverage area using a reflector antenna system requires specialized feed systems capable of supporting dual frequencies and providing dual polarization, and thus requires specialized feed system components. Also, the downlink signal, transmitted at high power (60-100 W) at the downlink bandwidth (18.3 GHz-20.2 GHz), requires low losses due to the cost/efficiency of generating the power and heat when losses are present.
These specialized feed system components include signal orthomode couplers, such as coaxial turnstile junctions, known to those skilled in the art, in combination with each feed horn to provide signal combining and isolation to separate the uplink and downlink signals. The current orthomode couplers are limited in their ability to provide suitable impedance matching between the downlink waveguide and the orthomode coupler over the complete downlink frequency bandwidth. Thus, there is a need in the art to provide a orthomode coupler that has better impedance matching between the orthomode coupler and the downlink waveguides. It is therefore an object of the present invention to provide an improved orthomode coupler having better impedance matching.
U.S. Patent application Ser. No. ""162, referenced above, discloses a coaxial turnstile junction for both satellite uplink and downlink signals that provides increased impedance matching between the downlink waveguide and the junction over the complete downlink frequency bandwidth. This junction has been effective for providing signal isolation by using coaxial waveguide chambers to isolate the uplink and downlink signals. However, other satellite applications require combining uplink and downlink signals that employ feed horns not based on coaxial signal separation. The invention satisfies that need.
In accordance with the teachings of the present invention, an orthomode coupler is disclosed for isolating and directing both satellite uplink and downlink signal, that provides for dual sense polarization. The coupler includes a first end that is in signal communication with an antenna feed horn. The coupler also includes a cylindrical outer wall defining a waveguide chamber that includes a first cylindrical section, a tapered section and a second cylindrical section. A plurality of symmetrically disposed downlink waveguides are positioned around the tapered section and are in signal communication with the waveguide chamber. Irises are provided at the connection between the downlink waveguides and the chamber for impedance matching purposes.
Satellite downlink signals propagate from the downlink waveguides to the feed horn through the waveguide chamber. Satellite uplink signals received by the feed horn are directed through the waveguide chamber and exit the coupler through the second cylindrical section to be sent to receiver circuitry. The dimensions of the irises and the flare angle of the tapered section are selected and optimized so that the downlink signal from the downlink waveguides is impedance matched to the waveguide chamber. The size of the second cylindrical section is selected so that the downlink modes do not propagate into the second cylindrical section.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings.